1. Field of the Invention
The present invention relates to a method and a device for making geophysical measurements within a wellbore.
2. Description of the Prior Art
Measurements performed within a wellbore usually fall into two main types.
The first type of measurement known as ultrasonic measurement is carried out by means of a tool which is lowered into the borehole at the end of a cable. The tool comprises one or a number of transmitters and one or a number of receivers in addition to means for turning-on the transmitters at suitable instants, and means for transmitting the signals picked up by the receivers to the land surface for recording and processing.
In the majority of instances, ultrasonic measurements are performed during displacement of the tool, that is to say when the tool is moving within the wellbore. The relatively low rate of displacement of the tool within the wellbore (a few meters per minute) does not produce any background noise in the receivers.
The receivers employed are broadly designated as transducers and more specifically as hydrophones but these devices in fact have the function of sensors which are responsive to vibratory pressure.
The sensors receive compressional waves and possibly also parasitic waves derived from the compressional waves and generated by the inhomogeneities of the surrounding medium. The duration of each useful signal received on the sensor, after each transmission, is of the order of a few milliseconds, taking into account the frequency of the signals transmitted.
Since the measurements are made during displacement of the tool, the traction cable of the tool is therefore continuously under tension and is wound continuously around a drum, the rotation of which is controlled, for example, by displacement means located at the surface.
Many types of ultrasonic measurement tools in current use make it possible to exclude waves which propagate within the mass of the tool itself and to produce acoustic paths having cylindrical symmetry about the axis of the wellbore.
The second type of measurement relates to a vertical seismic profile, an oblique seisxic profile or else a wellbore seismic profile, this second type of measurement being completely different from the first type which was recalled earlier by way of reference.
In fact, in the case of petroleum prospecting, for example, it is necessary to determine the nature and characteristics of the subsurface strata which surround the wellbore. To this end, seismic energy is generated at the earth surface and not within the wellbore by means of a detonation (explosive charge), a suitable shock impact (falling weight), or else a vibrator in contact with the ground surface. This seismic energy produces waves which propagate within subsurface strata and are received on geophones and no longer on hydrophones. The geophones are designed as velocimeters which are responsive to vibrational velocity and placed either on the surface, in which case they receive the waves reflected and/or refracted by the different subsurface strata, or within the wellbore, but in this case they must imperatively be in direct contact with the wall of the borehole in contrast to ultrasonic measurement in which the sensors are usually immersed in the fluid which fills the borehole.
In all cases, the geophones or velocimeters are responsive to vibrational velocity and must permit determination of all the types of seismic waves generated by the energy emitted at the surface. In fact, direct mechanical coupling between the transmission means and the earth surface gives rise to all types of waves since the emitted seismic energy necessarily encounters inhomogeneities in the subsurface formations under exploration. In consequence, it can no longer be considered sufficient either to receive one particular wave among others on the geophone or geophones or to accept a scalar quantity. On the contrary, the geophones must necessarily receive all waves (compressional waves or transverse waves) and in all directions in which they propagate in order to be able to distinguish all the possible directions of vibrational velocities at the time of processing.
Giving due consideration to the transmission means employed and the object to be achieved, the frequencies transmitted are within the range of a few hertz to a few hundreds of hertz (5 to 500 Hz, for example) and the duration of the useful signal is of the order of a few seconds.
A final point worthy of note is that, in the second type of measurement referred-to in the foregoing and in contrast to ultrasonic measurement, measurements are performed during upward travel of the tool whilst the geophone or geophones remain stationary with respect to the borehole wall and are applied against the wall in order to ensure a satisfactory coupling.
Subsequent processing of the signals received by the geophones makes it possible to obtain useful information on the subsurface strata traversed by the waves from the shot point to the geophones.
A number of different techniques are open to selection. It is possible to place geophones at either uniform or non-uniform intervals along the wellbore and to process information recorded on the different geophones. It is also possible to use a tool which carries one geophone and is stopped at each appropriate level of the well-bore during each measurement.
In the second type of measurement, the major difficulty arises from the fact that, up to the present time, no effective method has yet been found for obtaining a satisfactory coupling of the geophone with the borehole wall.
One attempt has already been made to overcome this difficulty and is described in French patent No. 1,169,871. The device of this prior art comprises a tool fitted with a cylindrical element which serves to maintain a geophone assembly at the desired height within the borehole and prevents the geophone assembly from rotating with respect to the tool by means of a key-type connection. The geophone assembly comprises two arcuate blade springs disposed symmetrically on each side of the tool which is centered within the borehole, the springs being joined to each other by means of pivots mounted on an intermediate member which is attached to the tool body. One of the blade springs is applied against the borehole wall whilst the other blade spring carries substantially at its vertex a shoe which is pivotally mounted at the midpoint of the spring. The outer face of said shoe is in contact with the borehole wall opposite to the wall against which the first spring is applied. A geophone is then fixed on the inner face of the shoe and is connected by means of a conductor cable to the amplifying equipment located within the tool body.
A tool of this type constitutes a significant improvement over the technique which consists in placing geophones on the ground surface or at intervals along the borehole wall. However, it does not offer a complete answer to the problem of direct coupling with the borehole wall.
There is in fact no direct contact between the geophone and the borehole wall since the geophone is mounted on the inner face of the shoe. Although the shoe is perfectly rigid and the tension of the springs does not permit any displacement of the shoe in sliding motion along the borehole wall while a measurement is in progress, it is nevertheless a fact that the parasitic waves generated by the emitted energy and propagated within the tool body, within the blade springs and within the shoe are received by the geophone. This arises from the rigidity of the tool and the shoe as well as the blade springs although to a lesser degree.
A further drawback arises from the fact that, in the case of a given measurement, the geophone is oriented in only one direction and is therefore capable of detecting only one type of wave.
A further disadvantage which appears to present the most crucial problem is that the blade spring which carries the geophone has only one degree of freedom as determined in the direction of deflection of the spring. The structure of the spring and the developed applied stresses do not permit flexibility of the spring in all directions. In consequence, there is no effective decoupling between the mass of the tool and the mass of the geophone and of the associated shoe. Moreover, the tool has a considerable mass in comparison with that of the geophone assembly. This mass introduces a parasitic effect which is greater as the connection between the tool body and the geophone assembly has greater rigidity.
It should also be noted that, by reason of the rigid connection between the mass of the tool and the geophone, the fact that the mass of the geophone is smaller than that of the tool prevents useful vibration of the geophone on reception of the high frequencies of the transmission spectrum. These high frequencies are thus eliminated and the result thereby achieved is much the same as if the tool body and the relative flexibilities of the borehole walls constituted a filter for these high frequencies.
The device employed in the second type of measurement also comprises tool-displacement means which are attached to the tool by traction and suspension means.
Irrespective of the nature of the tool traction and suspension means just mentioned and regardless of whether these means are flexible or rigid, or whether they consist of a traction cable or a rod, the measurements are performed one after the other in succession up to the full height of the borehole and in the following manner, for example.
The measuring tool is lowered to the desired depth or level, whereupon said tool is placed in position and maintained stationary against the borehole walls by means of retractable arms. The measurements are then performed in respect of the borehole level considered.
Since the next measurements are to be made at a higher level, the tool must be displaced in the upward direction by folding-back the retractable arms and by actuating the traction means. Another positioning operation is then performed in exactly the same manner as the operation mentioned above.
It is readily apparent that these sequential measurements also entail the need for sequential operation of the traction and displacement means which must be stopped during measurements and displaced between measurements. In the case of the traction cable, an additional operation is necessary. This consists in re-tensioning said cable at the end of each series of measurements since the cable is released throughout the duration of the measurements in order to ensure seismic decoupling of the tool with respect to the surface displacement means.
In consequence, it is apparent that there is a relatively substantial loss of time throughout the entire sequence of operations, which includes a certain number of successive adjustments for the different tool positions within the borehole.